The long-term objective of this project is a better understanding of the forces and interactions involved in protein folding reactions using cytochrome c as a model protein. A major hurdle in understanding the process of protein folding has been the difficulty of obtaining structural data on partially folded intermediate states. Hydrogen exchange labeling and rapid mixing methods developed in this laboratory in conjunction with two-dimensional NMR spectroscopy make it possible to observe the formation of H-bonded structure during refolding. Previous results on cytochrome c and other proteins have shown that this approach provides the spatial and temporal resolution to obtain a detailed structural and kinetic description of folding pathways. Further steps towards a complete mechanistic understanding of cytochrome c folding include the following: (1) the stability of folding intermediates and early folding events will be probed by H-exchange labeling studies under various refolding and labeling conditions; (2) the role of heme ligation and proline isomerization in folding will be explored by structural and kinetic studies on wild-type and mutant forms of cytochrome c; (3) circular dichroism and 2D NMR will be used to characterize synthetic peptides and proteolytic fragments derived from cytochrome c in a search for helical structure and helix-pairing reactions; (4) the importance of individual residues and interactions in cytochrome c folding will be explored by combining the structural approaches with site-directed mutagenesis. Additional plans include folding studies on bacterial cytochromes and H-exchange studies on the complex of cytochrome c with monoclonal antibodies in a search for antibody-induced conformational changes. The better structural understanding of protein folding provided by these experimental studies will be important for several basic and applied research areas. These include theoretical efforts to decipher the structural information encoded in amino acid sequences and biotechnology product design.